Once the most popular Hand Held computer Operating System in North America (and possibly elsewhere). Europeans like Symbian Os, and by 2010 North American's preferred Black Berry, Android Os and Iphone Os.
Machines that run Palm Os:
Palm Tungsten series
Sony Clie (all versions, and there are a bunch)
Some cell phones
Samsung Ixxx Series (OS 3.5.x and later)
"Dana" from Alpha Smart AlphaSmart.com 
Other relevant links/pages:
Quik Writing is an alternative input system to Palm Graffiti
news:comp.sys.palmtops.pilot
Programming on the Palm:
Quartus Forth (see also Sleepless Night Wiki)
GCC for the palm: www.palmos.com
Relevant Links to Palm pages:
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Biggest competing Operating System:
Android Os, the leading mobile OS as of 2013.
Symbian Os (in Europe)G
I guess we need to add Linux on Zaurus
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Palm complaints:
It'd be nice if the palm had fixed-width fonts. The three fonts that come with the Palm Pilot are proportional fonts. Reading code in a proportional font just seems uncomfortable.
<i>The PEdit editor has several fixed-width fonts of various sizes.</i> -- Kris Johnson
I think that Palm Os is the current best example of what an ideal end user OS should look and act like. Many people have trouble with windows, but anyone can figure out a Palm Pilot.
<i>Is PalmOS still available? It seems everything has gone the way of smartphones and the web... What about those of us who simply want to organize our pocket information?</i>
In 2010, there is nothing much (if anything) on the market that is pure plain Palm OS. A couple of companies seem to have Palm OS 5 Garnet runtimes or emulators in their newer multi-tasking platforms in order to be able to run "Classic" Palm OS applications made for the 68k or ARM devices: webOS (Web Os, from Palm, Inc. offers Motion Apps Classic www.motionapps.com ) and ACCESS Company's Access Linux Platform ( www.accessdevnet.com ).
<i>Aceeca, a company in New Zealand, still (2014) seems to be making Palm OS (Garnet) 5 PDAs for industrial use (aceeca.com )</i>
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See original on c2.com